INFLUENCES OF LIGAND AND OF ZEOLITE TOPOLOGY ON THE STRUCTURE OF CO-II SCHIFF-BASE CHELATES IN FAUJASITE-TYPE ZEOLITES

The reaction in dehydrated faujasite zeolites of two different topologies between charge-compensating Co-II cations and five different Schiff base ligands was studied by EPR and vis-near-IR spectroscopy. Tetradentate as well as pentadentate ligands were used for this comparison, while the behavior of a faujasite with cubic symmetry (zeolite Y) was compared to that of EMT, a faujasite with hexagonal symmetry. After reaction of the Schiff bases salen (bis(salicylaldehyde) ethylenediimine) (1) and acacen (bis(acetylacetone) ethylenediimine) (2) with CoNaY zeolite, low-spin square-planar Co-II was observed in low concentrations in [Co-II(acacen)]NaY. In [Co-II(salen)]NaY, formation bf planar complexes is doubtful. Use of these tetradentate ligands for oxygen activation in zeolite Y results in low concentrations of active, oxygen-binding Co-II. Incorporation of a nitrogen base into these Schiff bases results in the pentadentate ligands smdpt (bis(salicylaldehyde) methylnitrilodipropylenediimine) (3) and amdpt (bis(acetylacetone) methylnitrilodipropylenediimine) (4). Use of these ligands greatly raises the fraction of the Co-II participating in the oxygen activation. When the Co-II form of the hexagonal faujasite NaEMT is loaded with smdpt (3), up to 25% of the total Co-II reversibly binds dioxygen. Intrazeolitic Co-II was also reacted with pyren (bis(2-pyridinecarboxaldehyde) ethylenediimine) (5), a ligand which occupies a position structurally intermediate between Schiff bases and bipyridine. Different EPR parameters were observed for [Co-III(smdpt).O-2(-)] occluded in cubic faujasite (NaY) and hexagonal faujasite (NaEMT), reflecting the influence of the zeolite topology on complex formation. The dioxygen sorption of these new materials was studied quantitatively.